Through our Natural History of Stroke study(Protocol No. 01-N-0007; Clinicaltrials.gov No. NCT00009243) we have collected data on more than 2,400 study participants in order to learn more about stroke and obtain information that may serve as the basis for future investigations. This protocol has allowed us to 1) establish a registry of patients with cerebrovascular disease (stroke); 2) characterize the natural history of acute stroke and transient ischemic attacks (TIA) an interruption of blood flow to the brain that causes stroke symptoms for a short period of time); and 3) evaluate the data to generate ideas for future studies. MRI has improved our ability to diagnose and stratify patients with acute stroke by providing highly sensitive and specific markers of the disease. Imaging based phenotypes of stroke increase objectivity, however they remain a gross oversimplification of the complex biological system set in motion by a stroke. Next generation sequencing, with unprecedented improvement in throughput and speed, provides an opportunity to probe the complex biological response to stroke in patients stratified using acute MRI. Based on the premise that the biology responsible for the imaging abnormalities will be reflected in differential gene expression and micro RNA in peripheral blood, next generation sequencing will be used to identify and characterize the biological systems relevant to the imaging phenotype. A systems biology approach will be developed to better describe stroke, and hopefully, better differentiate those patients in who we can expect a favorable response to an intervention, from those at risk of further deterioration. We have created a unique and comprehensive dataset of acute stroke MRIs (Lesion Evolution in Stroke and Ischemia On Neuroimaging LESION) analysis. This dataset of acute stroke MRI affords unprecedented opportunities for robust statistical analysis of the incidence of ischemia and reperfusion as related to time from stroke onset. Imaging based predictors of stroke outcome and response to therapy are necessary for the utility and validation of imaging biomarkers in drug development. Useful models are those that can distinguish patients destined for good outcomes versus poor outcomes, those who received effective therapy from those who did not, and treatment responders from non-responders. We are investigating several predictive models. These prediction models may be useful for the development, selection and use of acute therapies. We found that change in lesion volume from pre-treatment DWI to post-treatment FLAIR can discriminate between patients destined for good and poor outcomes when treated with effective acute stroke therapy, i.e., intravenous tPA. Thus, lesion volume change may be a useful marker of clinical response in the stroke therapy development. As part of the Stroke Branch, the Neuro Vascular Brain Imaging Unit (NVBI) heavily utilizes the data collected through the Natural History of Stroke protocol. The NVBI has taken on the big data challenge of co-registering and quantitatively processing the MRI scans acquired as part of the natural history study. In doing so the NVBI has created an interface, referred to as the pipeline, that leverages the massive amount of information contained in the natural history dataset to perform scientific hypothesis testing. The pipeline is available to the entire Stroke Branch as a research resource. NVBI has been using the natural history imaging pipeline to perform quantitative blood-brain permeability imaging. Using a novel post processing method, a measure of blood brain barrier integrity can be extracted from the existing dataset. This has lead to new insights into the pathophysiology of acute and chronic cerebrovascular disease that has the potential to influence clinical care. Future directions include expanding the natural history study to collect data at chronic time points as well as the introduction of new imaging methodologies such as pH-weighted imaging. In addition to the Natural History of Stroke protocol, we have an ongoing clinical trial - MR WITNESS: A Study of Intravenous Thrombolysis With Alteplase in MRI-Selected Patients (NIH Protocol No. 11-N-N019; Clinical Trials.gov No. NCT01282242). This clinical trial was jointly developed and is jointly led by investigators at Massachusetts General Hospital and our intramural stroke program at NINDS. We are doing this research study to find out if Activase (also called alteplase or rt-PA) can safely be given to people with an acute ischemic stroke when their stroke onset was not witnessed making them ineligible for standard thrombolytic (clot busting) therapy. We also want to find out if rt-PA can help people recover better from their stroke. The purpose of this study is to: 1) see if it is safe to give intravenous (IV) rt-PA to people with unwitnessed stroke but with MRI evidence of early ischemic stroke, 2) see if rt-PA is effective if given to people who are selected for treatment based on MRI evidence of an early stroke, and 3) get information about this new MRI diagnostic methods for guiding stroke treatment. To date, our intramural site has enrolled 24 subjects into this study.